Hollow filaments

ABSTRACT

A HOLLOW FILAMENT OF SYNTHETIC POLYMER MATERIAL IS DISCLOSED CONSISTING OF A SHEATH SURROUNDING A LINGITUDINALLY EXTENDED CAVITY. THE SHEATH COMPRISES A CONTINUOUS PHASE POLYAMIDE POLYMER CONTAINING DISPERSED THEREIN A POLYESTER IN THE FORM OF MICROFIBERS LYING PREDOMINANTLY IN THE DIRECTION OF THE AXIS OF THE FILAMENT.

Jan. 26, 1971 J. OPFELL I HOLLOW FILAMENTS Filed Aug. 17. 1967 INVENTORJAMES E.OPFELL Raimi 61' Huh/ W F'IG.| I.

AT TOR N EY United States Patent (1) 3,558,420 HOLLOW FILAMENTS James E.Opfell, Colonial Heights, Va., assiguor to Allied Chemical Corporation,New York, N.Y., a corporatloll of New York Filed Aug. 17, 1967, Ser. No.661,348 Int. Cl. D01d 5/24 US. Cl. 161176 3 Claims ABSTRACT OF THEDISCLOSURE A hollow filament of synthetic polymer material is disclosedconsisting of a sheath surrounding a longitudinally extended cavity. Thesheath comprises a continuous phase polyamide polymer containingdispersed therein a polyester in the form of microfibers lyingpredominantly in the direction of the axis of the filament.

BACKGROUND OF THE INVENTION This invention relates to novel hollowsynthetic filaments and to the production thereof. More particularly, itrelates to the production of said filaments by extruding a moltensynthetic polymer material through a spinnerette containing at least oneunobstructed orifice followed by controlled cooling of the freshly spunfilaments to effect solidification thereof.

It is recognized that hollow synthetic filaments have certain advantagesover solid filaments having the same outside diameter. Some of theadvantages which hollow filaments have compared to solid filamentsinclude improved insulating properties, increased bouyancy, and greatercovering power (e.g., in carpet yarn) per unit weight. Hollow filamentscomposed of polymer blends also have less tendency to fibrillate underflexing conditions than corsponding solid filaments.

However, it has proved to be extremely difficult to manufacture hollowfilaments in a commercially feasible manner by melt-spinning.Considerable time, effort and money have been spent on attempts to adaptexisting methods to the production of hollow filaments on a commercialscale. Processes which have been devised for this purpose havenecessitated the use of special and often expensive processingconditions and equipment. Such improvements as have been made havegenerally been related to the spinnerette. Unfortunately, thespinnerettes that have been designed thus far are difficult to constructand are subject to frequent breakdowns which may be attributed at leastin part to their complex construction.

One type of spinnerette employs orifices containing an internalobstructing member which causes the orifice to function as an annulus,said obstructing member being joined to the spinnerette body by internalsupport members upstream from the extrusion face of the spinnerette.This type of spinnerette and others which have been designed to prod-neehollow filaments are not only hard to make but are extremely difiicultto maintain in a good state of repair and cleanliness.

Other spinnerettes which have been studied employ a multitude of simple,unobstructed orifices grouped in a perimeter. They are so closely spacedthat, upon extrusion, molten polymer emerging from each of the orifices"coalesces with extrudates from the adjacent orifices of the group toform a continuum of the polymer substrate which, after rapid cooling,forms a hollow-shaped filament. The filament is hollow because the areaencompassed by the perimeter of orifices contains no openings andthereby blocks passage of polymer. These spinnerettes require very closespacing between adjacent orifices to permit coalescence of extrudatestreams with the result that the thickness of the intervening metalbetween ori- Ice fices is so small as to cause structural weakness anddifficulties of fabrication. The weakened nature of these spinnerettesis particularly significant in the melt-spinning of synthetic fibersbecause the high extrusion pressures required will often causedistortion or actual rupture of spinnerettes of inadequate strength.Another disadvantage of orifices which are too closely spaced in forminga perimeter is that polymer coalescence may occur too close to the faceof the spinnerette, thereby preventing the entrance of air into thehollow cavity of the filament. The resultant vacuum within. the filamentcauses internal coalescence of the molten polymer, thus minimizing oreliminating the central cavity.

SUMMARY -OF THE INVENTION Therefore, it is an object of the presentinvention to provide a filament having an internal cavity extendingalong its length.

Another object is to provide an improved melt-spinning process for theproduction of hollow synthetic filaments.

Yet another object of the present invention is to provide amelt-spinning process for the production of hollow synthetic filamentsusing a spinnerette having unobstructed orifices which provide at leastone gap in an otherwise continuous periphery of openings encompassing apolymer occluding area.

Other objects and a fuller understanding of the present invention may behad by reference to the following description, drawings, and appendedclaims.

The objects of this invention are accomplished in general by:

(a) Extr-uding a molten synthetic polymer composition having a die swellfactor greater than about 2.5, and preferably between about 2.5 andabout 10, through a spinnerette containing at least one circumscribingorifice arrangement. Said orifice arrangement comprises at least oneunobstructed orifice and defines, without completely surrounding, apolymer occluding area which is devoid of orifices. This occluding areacommunicates with the area outside the orifice arrangement by means ofat least one passage through said arrangement. The average diameter, Dof the polymer occluding area is greater than AD wherein A is the dieswell factor and D is the maximum orifice width in a direction radial tothe center of said occluding area; and

(b) Cooling the polymer extrudate to effect solidification thereof.

In the accompanying drawings:

FIG. 1 is a vertical sectional view of part of a spinnerette useful inthe practice of the present invention;

FIGS. 2 to 8 are plan views of spinnerette orifice arrangementsaccording to the present invention comprising more than one discreteunobstructed orifice, each of said orifice arrangements having aperimetric configuration about a polymer occluding area. The spinneretteillustrated in FIG. 8 is disclosed and claimed in copending U.S.application Ser. No. 687,170, :filed Dec. 1, 1967.

FIG. 9 is a plan view of a spinnerette orifice arrangement according tothe present invention comprising a single discrete orifice conformedinto an encircling configuration about a polymer occluding area.

FIG. 10 is a partial sectional view showing the coalescence of polymerextrudates downstream from the spinnerette; and

FIGS. 11 and 12 are cross-sectional views of hollow filaments producedin accordance with the present invention.

In FIG. 1, spinnerette 1 has an upstream face 3 and downstream extrusionface 5. Spinnerette 1, being typical of those employed in melt-spinningof synthetic fibers, is generally about 0.2-1.0 inch thick and may be ofmonolithic or laminated construction. It is generally made of steel orother high strength metal or alloy. Counterbore depression 7 may beformed in the upstream face of spinnerette 1 in order to minimize thelength of capillaries 9 leading to orifices 11 which define polymeroccluding area 13.

The capillaries of spinnerettes useful in the practice of the presentinvention are generally of constant cross section throughout theirlength, i.e., the capillary wall is a cylindric surface as may be formedby circular movement of a straight line parallel to a given fixedcentrally located straight line. The capillary may however, be slightlychamfered at either or both ends. The length of a typical capillary ispreferably between about 8 and about 70 mils (l mil=0.00l inch). If thecapillary length is appreciably below 8 mils, the spinnerette plate maybe undesirably weakened, while capillary lengths substantially greaterthan about 70 mils may cause difficulties in obtaining satisfactoryhollow filaments by the process of this invention. The capillaries maybe made by standard techniques which include: drilling with rotarydrills; drilling with pneumatic devices or electrodes; punchingtechniques; and insertion of shaped wires into differently shapedcapillaries (as disclosed in US Pat. 3,174,364). The capillaries arepreferably perpendicular to the extrusion face but may be angled todirect extrudate streams toward one another for improved coalescence.

In FIG. 2, portion 15 of spinnerette .1 contains a circumscribingarrangement of six individual orifices 17, each having diameter, DOrifices 17 are perimetrically disposed in symmetrical arrangement aboutcentral point 19, thereby defining central polymer occluding area 21,having average diameter D taken as the diameter of the circle whichtangentially contacts the orifices at points closest to central point19. Spacings 23 provide passages through which the occluding area 2.1communicates with the rest of the spinnerette face. As will be explainedmore fully hereinafter, adjacent polymer streams coalesce just belowthese passages. If, for a given set of extrusion conditions, thefollowing relationship holds:

then separate polymer streams from adjacent orifices will coalesce toform the desired hollow filaments.

In preferred embodiments of the present invention, D is greater than 2ADIn FIGS. 3 through 8, D is the maximum distance within an orifice in adirection radial to the center of the polymer occluding area. D may betaken as the average length of all the straight lines which can be drawnbetween the inside walls of the orifices through the center of thepolymer occluding area.

FIG. 9 illustrates an orifice arrangement consisting of a singlediscrete orifice having a spiral shape. For the purpose of ascertainingthe value of D since there is no center of symmetry, a center of gravitymay be chosen for the configuration, and D may be taken as the averagelength of straight lines which can be drawn through said center ofgravity between opposing orifice boundaries.

The individual orifices may be of any cross-sectional shape, e.g.,circular, rectangular, crescent shape, or other curvilinear or polygonalshape. Elongated orifices are preferred, especially those having a shapefactor of from 1.4 to 20. The shape factor is defined as the ratio ofthe longest straight line which can be drawn within a cross section ofthe orifice to D Accordingly, circular orifices are not preferred sincethe shape factor in this case is one. The cross-sectional area of eachorifice is preferably between about 12x10" and about 25 10 in. In thecase of circular orifices, this would correspond to a diameter ofbetween about 4 and about 55 mils. The orifice arrangement, which mayconsist of one, two, three, or more discrete orifices serving to producea single hollow filament, generally has an axis of symmetry, andpreferably a point of symmetry on the face of the spinnerette.

A spinnerette may contain any feasible number of 4 such orificearrangements. The hollow, centrally positioned cavity of the resultantfilament is produced by the absence of polymer extrudate within theperimeter of the orifice arrangement.

There is at least one and preferably several passages leading fromwithin the otherwise complete annulus formed by the circumscribingorifice arrangement. The passages between the orifices, consistingessentially of material of which the spinnerette is composed, are madesmall enough to permit coalescence of the individual extrudate streams,yet far enough apart to preserve the strength of the spinnerettestructure and to permit the atmosphere or gas to enter betweenindividual extrudate streams prior to their coalescence. In general, theminimum distance of separation between the orifices should be greaterthan 3 mils to insure spinnerette strength, and less than about 15 milsto secure satisfactory interstream coalescence. Such spinnerettes aremade useful in the practice of the present invention by virtue ofoperating conditions which provide a polymer die swell factor greaterthan 2.5.

The die swell factor of filament-forming synthetic polymers is a knownmeasureable value and is defined as the ratio of the maximum diameter ofthe extrudate stream to the diameter of the orifice opening whenemploying a circular orifice removed from the influence of coalescingstreams. The extrudate stream diameter may be measured by photographicor visual methods. In the case of noncircular orifices the die swellfactor is measured on a circular orifice having the same area as saidnoncircular orifice, all other conditions being the same.

The die swell factor in the course of melt-spinning of homogeneousthermoplastic polymers is dependent upon several factors. It can bedetermined by simple tests, and the spinnerette for producing the besthollow core filaments selected on the basis of the factor as determined.Alternatively, the die swell factor can be varied to accommodate aparticular spinnerette in accordance with known procedures for varyingsaid die swell factor. Thus, holding all other variables constant, thedie swell factor can be increased by:

(a) Decreasing the speed of the take-up roll.

(b) Decreasing the orifice extrusion temperature.

(0) Decreasing the residence time within the capillary (i.e., increasingthe throughput). In this connection it should be noted that increasedthroughput even when compensated by increased take-up speed to maintainconstant denier of the wound filament, still increases the die swellfactor.

(d) Increasing the polymer melt viscosity; and

(e) Decreasing the diameter of the orifice.

The site of maximum swelling or stream diameter is generally within thefirst inch downstream from the spinnerette. The actual distance,however, tends to increase with higher throughput values.

In the case of the usual production of synthetic fibers by melt-spinningfrom polymers such as polyamide, polyesters, and polyolefins, the dieswell factor rarely exceeds 1.5. The occurrence of die swell factorsabove 1.5 has generally been considered undesirable since it is normallyassociated with the production of filaments of varying diameters alongtheir lengths. The phenomenon of the swelling or expansion of a freshlyextruded polymer stream is attributable to the elastic characteristicsof the polymer and to the tendency of the aligned polymer molecules tobecome disoriented upon emerging from the capillary.

In accordance with the present invention, when synthetic polymers arespun under conditions such that the die swell factor exceeds 2.5, asufficiently large bulge is formed in the still molten extrudate topermit location of the separate orifices sufiiciently far apart topermit ambient air or gas to enter the space between individualextrudate streams prior to coalescence, thereby preventing collapse ofthe individual streams into a solid filament. The location of theorifices at greater distances of separation has the further advantage ofgiving increased strength to the spinnerette. In prior art methods,deficiencies in spinnerette strength have manifested themselves in abulging or actual rupture of the central polymer occluding area.

With polymers spun under conditions such that the die swell factor isappreciably below 2.5 or above 10, the behavior of the molten extrudatebecomes such that the spinnerettes described herein could not be usedfor making hollow filaments. It is possible, however that the polymercould be spun by modifying the spinning conditions so as to bring thedie swell factor within the desired range.

Thermoplastic polymers suitable for use in the present inventioninclude:

(a) Polyesters such as polyethylene terephthalate and polyhexahydrop-xylylene terephthalate.

(b) Polyamides such as polyhexamethylene adipamide (nylon-66) andpolycaproamide (nylon-6).

(c) Polyolefins.

(d) Polyurethanes.

(e) Polyesteramides.

(f) Polyethers; and other synthetic polymers and mixtures thereof whichmay be spun under conditions which ensure a die swell factor within thepreferred range. When only one polymer entity is employed it must befiber-forming. If a mixture of polymers in which one polymer is in theform a dispersed phase within a continuous phase of the other polymer,the dispersed phase is not necessarily fiber-forming.

Suitable nylon polymers having satisfactory fiber-forming propertiesgenerally have molecular weights which are preferably in the range ofabout 15,000 to 40,000. Such polymers will have formic acid relativeviscosities of 30 to 150, and preferably between 30 and 100, asdetermined at a concentration of 11 grams of polymer in 100 ml. of 90%formic acid at 25 C. (ASTM D-789-62T).

Suitable polyester polymers having satisfactory fiberforming propertiesgenerally have a reduced viscosity above 0.50. The reduced viscosity ofpolymers useful in the compositions employed in this invention isdetermined by viscosity measurements carried out at 25 C. on a 0.5% byweight solution of polymer in purified orthochlorophenol containing 0.1%water. Employing a standard Cannon-Fenske 150 bore viscometer, the flowtime of the polymer solution (t is measured relative to the flow time ofthe solvent (t and the reduced viscosity iscalculated using thefollowing equation:

N red: (11,-1 /C where:

N red=reduced viscosity C=concentration of dissolved polymer in grams/100 ml. n,.= relative viscosity=tp/t We have found that a particularlypreferred class of thermoplastic compositions for use in the practice ofthis invention comprises heterogeneous compositions consisting of adispersion of discrete regions or particles of one polymer within acontinuous phase of a second polymer. Such heterogeneous compositionsafford unexpectedly higher die swell factors under conditions which arenot adverse to the general requisites of good spinning practice. Forexample, whereas many single-component polymer systems require lowerthan normal extrusion temperatures to obtain an adequate die swellfactor, for the purpose of this invention the aforesaid heterogeneouscompositions provide suitable die swell factors at higher temperatures,a factor which also favors interstream coalescence and good fiberformation. Although it is not intended that the present invention bebound by theory, it is felt that the high values for the die swellfactor obtained with such heterogeneous systems may be due to thetendency of the minute globules of dispersed phase polymer, which emergefrom the orifice in an elongated molten condition, to reassume aspherical shape in order to decrease their free energy content. Intending toward the spherical form, the dispersed phase polymer globulestend to slow down the polymer stream or cause a shortening thereof,which causes a bulge immediately beneath the spinnerette. Particularlydesirable heterogeneous polymer compositions comprising polyesterparticles dispersed in polyamides are disclosed in copending US. patentapplication Ser. No. 368,028, filed May 18, 1964, in the name of I. C.Twilley, now Pat. No. 3,369,057.

FIG. 10 illustrates the melt spinning process of this inventionemploying a heterogeneous polymer composition. As depicted therein, amolten composition comprising a continuous phase polymer 25 anddispersed discontinuous phase polymer 27 is extruded through closelyadjacent orifices 29 and 31 of spinnerette 33. Under the shearconditions within the capillary a dispersed polymer globule assumes anelongated form 35. Upon emerging from the capillaries, the elongateddispersed polymer globule assumes the more spherical form 37.Concurrently, the diameter of the extrude composition expands to amaximum value at a site 39 which leads to coalescence of the two streamsof polymer emerging from spinnerette orifices 29 and 31. Downstream fromthis point, the generally spherical globules again become elongated asexemplified by globules 37a, 37b and 37c. At the same time, the diameterof the extrudate (now a hollow filament) decreases as it comes under theinfluence of drawing and stretching forces exerted by the take-up roll.

In the preferred heterogeneous compositions, both polymers should becapable of elongation at temperatures above their melting point andpreferably also capable of elongation under cold-drawing conditionsbelow the melting point. The dispersed polymer need not be fiberforming,i.e., capable of forming a continuous filament of useful strength by astandard melt-spinning operation. It should also be sufiicientlyincompatible with the continuous phase polymer so as to remain adistinct, discontinuous phase dispersed within the continuous phaseduring melt-spinning. It is preferable that the dispersed phase polymerhave a melting point in the range of between about 50 C. below to aboutC. above the melting point of the continuous phase polymer. Aparticularly preferred range for the polyester-polyamide blendsdescribed in the aforementioned Twilley application is for the meltingpoint of the polyester to be between about 10 C. to about 90 C. higherthan the melting point of the polyamide. For the purposes of the presentinvention, polymer melting points may be determined by using a hot-stagemicroscope. For those polymers having indistinct melting points, themelting point is taken as the temperature at which the polymer becomesfiowable at atmospheric pressure.

Hollow filaments prepared from said preferred heterogeneous polymermixtures are found to present a visual appearance characterized ashaving aesthetically pleasing sparkle or glitter. The novel filamentsare also found to have improved strength and rigidity in view of thereinforcing eifect of the dispersed phase polymer which, in the drawnfiber is in the form of microfiber reinforcing elements dispersed withinthe continuous phase polymer structure in a direction generally parallelto the axis of the filament.

Polymer die swell factors greater than 2.5 may also be secured byincorporating a gas or gas-forming material within the polymer meltprior to extrusion. Said gasforming material may be in the nature of agaseous or volatile liquid material which, upon extrusion and consequentsudden decrease in pressure, will have a tendency to increase in gaseousvolume. Such gaseous or gas-liberating substances may conveniently beemployed in amounts ranging from about 0.1% to about 2% by weight basedon the total weight of the polymer composition prior to extrusion. Theremay also be incorporated within the polymer composition finely dividedand dispersed particles which serve as nucleation sites for bubbles andwhich therefore control the uniformity of bubble size and distributionupon extrusion. The resultant hollow filament obtained when employingpolymer compositions containing gas-forming agents may or may not retainbubbles, depending upon the rate of bubble formation and quenching ofthe extrudate in the course of forming the solidified hollow filament.Suitable gas-forming agents include, for example, methanol, methylenechloride, hexane, benzene, fluorocarbons, water, and the like.

For extrusion, the molten polymer composition is metered to thespinnerette at pressures which vary somewhat with the composition, butgenerally are in the range of about 1,000 to about 10,000 p.s.i.g. Thecomposition is preferably passed through a filtration device such as asand pack prior to entering the capillaries. The temperature of thepolymer mixture at the spinnerette is maintained generally at betweenabout C. and about 50 C. above the melting point of the highest meltingpolymer component of the composition. In the case of heterogeneouspolymer blends, melt-spinning is preferably conducted under conditionswhich are interadjusted to maintain the value of the dimensionless ratioR/Pu in the range between about 0.0001 and 0.002, where R is the rate ofthroughput per orifice, P is the orifice diameter or average orificediameter in the case of noncircular openings (which may be approximatedby the average diameter of a series of circles inscribed within theorifice on the plane of the spinnerette), and u is the viscosity of themolten polymer blend at the extrusion temperature all factors beingexpressed in self-consistant units. In this manner, uniform hollowfilaments will be consistently produced. If the value of R/Pu is greaterthan 0.002, then discontinuities or drips may occur during spinning. If,on the other hand, the value of R/Pu is less than 0.0001, then a pulsingeffect may occur, which results in erratic control of filament denier.

In accordance with the usual melt-spinning techniques, the plasticextrudates of this invention are cooled to solidification by contactwith a gaseous medium of controlled thermal properties. In suchprocesses, the spinnerette usually forms the upper end of an enclosedcylinder known as a quench stack wherein controlled cooling to solidifythe extruded polymer is effected. Cooling is usually brought about bycontacting the extrudate with air or other gas which is chemically inertto the polymer under controlled conditions of temperature, fiow rate,and flow pattern. The cooling gas flow pattern may be cocurrent,countercurrent, or crosscurrent to the filaments. Gas temperatures maybe as high as about 50 C. or more above the temperature of the extrudedpolymer at the face of the spinnerette and may decrease in apredetermined manner down the stack thereby controlling the length oftime or traveling distance during which the extrudate is in a flowableform. Gas flow rates in the range of about to about 1,000 ft. min. maygenerally be employed.

The solidified filaments are removed from the quench stack by means of adriven take-up roll which is generally located below the quench stack.By suitable adjustment of the peripheral speed of said take-up roll,controlled elongation of the molten extrudate stream prior tosolidification may be effected, the extent of said elongation beingknown as stack draw-down." The speed of the take-up roll may also effectthe die swell ratio. In order to obtain satisfactory coalescence so asto produce hollow fibers, it is preferred that the polymer remainflowable for at least 5 mm. downstream from the spinnerette orifices andthat the stack drawn-down be maintained between about 10 and about1,000.

The extrudate, while still molten may be subjected to ultrasonic energy,electrostatic radiation or other physical treatment to securespecialized effects such as periodically closing the hollow filamentcore to form cells, imparting a crimpability to the filaments, and othereffects.

After being taken-up below the quench stack, the filaments may besubjected to a drawing operation at temperatures below their meltingpoint to confer molecular orientation along the filament axis andincrease the strength of the filaments. Dra w ratios of between about 2and about 12 have been found to impart maximum yarn strength. Theoptimum draw ratio, however, will vary with the selected polymer orpolymer mixture. The drawing operation is preferably carried out bystandard methods which may use a draw pin to localize the region ofdrawing. The yarn may be drawn in either a single or successive stages,and at least one of said drawing stages may be carried out while theyarn is heated, e.g., to a temperature between about C. and about C. fornylon 6 yarns. The optimum temperature will, of course, vary with theselected polymers. The heat can be applied to the yarn by known meanssuch as stationary or rotating contact heaters, steam chambers, heatedliquid sprays or baths, infrared, radio frequency heating, and othermeans.

Finishing compositions comprising lubricating ingredients and/or Wax canbe applied to the yarn prior to drawing to facilitate the drawingoperation. Prior to packaging, or in subsequent operations, the drawnyarn can be subjected to annealing at constant length or to relaxationwith controlled shrinkage in order to reduce the shrinkage and/ orultimate elongation of the yarn. One suitable method of effecting suchtreatments is disclosed in Wincklhofer US.

Pat. 2,859,472, issued Nov. 11, 1958. The drawn yarn can be subjected totreatment with ionizing radiation, ultrasonics, crosslinking agents suchas formaldehyde and isocyanates, and other finishing treatments tosecure various desired effects, none of which, however are to beconsidered as limiting the scope of the invention.

The product of this invention is a synthetic filament consistingessentially of a sheath and an internal, longitudinally extended cavity.The cavity may occupy between about 10% and about 80% of the entirecross-sectional area of the filament. The sheath may be of circular ornoncircular shape. The cavity may be centrally or eccentrically disposedwith respect to the filament axis, and the peripheral contour of thecross section of the cavity may be of the same or different from theshape of the cross-sectional periphery of the sheath. The shapes of boththe cavity and sheath will, however, be essentially constant along thelength of the filament. The sheath portion, consisting of the extrudedpolymer composition, will contain, in the case of heterogeneous polymerblends, the dispersed polymer in the form of microfibers. In preferreddrawn filaments these microfibers will have an average diametergenerally not greater than about 1 micron, and an average length of atleast about 5 times their diameter. The microfibers lie predominantly inthe direction of the filament axis, and there may be from 500 to morethan 150,000 microfibers per 1,000 square microns of transverse area ofthe drawn filament. The presence of microfibers in the hollow filamentsof the present invention imparts improved bending modulus, or stiffnesscharacteristics which are especially appreciated in textile apparel andin carpeting where improved resiliency is usually a desirable feature.

FIG. 11 represents a cross-sectional view of filament 41 comprisingannular round sheath portion 43 of uniform thickness and circular cavity45 which is substantially centrally disposed with respect to sheath 43.

FIG. 12. illustrates a cross-sectional view of a filament 47 of thisinvention containing dispersed microfibers 49 within continuous phasepolymer 51 of the noncircular sheath 53. Cavity 55 also is noncircular,but it is differently shaped than the outer periphery of the sheath 53.

The hollow filaments of this invention may be made to contain variousadditive ingredients which impart specialized properties. For example,ingredients which may be added to the filaments either by incorporationwithin the polymer prior to spinning, or by aftertreatment of the yarnor fabric include: flame retarding agents such as compounds of antimony,phosphorous, and halogens; delustrants such as titanium dioxide, calciumacetate and other opaque metal compounds; antistatic agents; adhesionpromoting agents such as isocyanates and epoxides, heat and lightstabilizers such as inorganic reducing ions, metal ions such asmanganese, copper and tin, phosphites, and organic amines such asalkylated aromatic amines and keto-aromatic amine condensates; thermallystable organic and inorganic pigments; fluorescent agents andbrighteners; crosslinking agents; bacteriostats such as phenols andquaternary amines; colloidal reinforcing particles; antisoiling agentssuch as colloidal silica and boehmite; dyeing modifiers; lubricants suchas molybdenum disulfide and silicones; lasticizers; dispersing agents tofacilitate and maintain dispersion of one polymer of a heterogeneousblend within the other; and other additives and treatments. Gaseousmateirals may also be incorporated within the internal cavity of thefibers of this invention. Such additives may be incorporated by servingas the quenching media for the filaments. In this respect, fibers ofthis invention may serve asuseful containers for the controlled use ofsuch gases.

Thermally stable flow controlling agents or surface active agents whichdecrease surface free energy may be included within the polymer prior toextrusion to increase the extent of extrudate coalescence, and therebyimprove the circularity of the sheath structure where such is desired.Examples of such agents include metal salts of long chain aliphaticcarboxylic acids, long chain aliphatic alcohols, long chain aliphaticamides, and other surface active agents.

The filaments of the present invention are useful in numerous textileapplications in the form of monofilament and multifilament yarn and tow,cords, and staple spun yarns. The filaments may be blended with otherfibrous materials, and may be employed in crimped or uncrimpedconditions. Typical textile applications include apparel products suchas woven suitings, shirtings, sheeting and lingerie, tricot, circularknitted fabrics, broadcloths, satins, and the like. In view of theirrelatively high stiffness, strength, and low weight, the blended polymerfilaments of this invention are further useful in textile applicationssuch as sewing thread, tire cord, fiber-reinforced laminates upholstery,carpeting, drapery, curtains, ducks, parachutes, reinforced belts andhoses, marine lines, ropes and netting, and other applications. Thefilaments may be admixed with solid core filamentary structures ofvarious modified cross section, of the same or different denier and thesame or different chemical composition to produce various specialeffects.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following examples, allparts and percentages are by weight unless otherwise indicated.

Example 1 A thermoplastic polymer composition comprising a uniformheterogeneous molten blend of 30% polyethylene terephthalate of 0.7 6'reduced orthochlorophenol viscosity, and 70% polycaproamide of 50 formicacid relative viscosity is prepared in the presence of trace amounts ofsebacic acid which serves as a chain length terminator, whereby fewerthan 40% of the terminal groups are amino groups. The blendedcomposition is prepared by meltblending the two polymer components underconditions of high shear so that the dispersed polyester in thecomposition has an average particle size of 0.8 micron. The compositionis then extruded at 280 C. at a rate of 4.6 pounds per hour, and aresidence time within the capillary of 3.8 seconds through a spinnerettehaving 16 groups of rectangular orifices essentially as shown in FIG. 4wherein each orifice is 0.004 inch wide (average diameter) and 0.032inch long, and the capillaries are 0.020 inch deep. The threerectangular orifices, having a shape factor of 8.0, are arranged aboutthe center of a circle having a radius of .020 inch. The effective valueof D is about 0.050 inch. The closest spacing between the orifices is0.004 inch. Under these conditions, the polymer has a die swell factorof 3.5 as measured on a circular orifice having a diameter of 0.013(which provides the same orifice area as one of the rectangularorifices). Upstream from the surface of the spinnerette is asandand-screen filter pack. The pressure on the polymer at the upstreamface of the spinnerette is about 1900 p.s.i.g. The value of R/Pu is0.00137. The value of AD is 0.004 3.50.014".

The filaments are extruded into an atmosphere of air at 72 C., flowingconcurrently to the extrudate at a rate of 20 cubic feet per minute. Thesolidified yarn is wound up at a rate of 1170 feet per minute. The yarnthus obtained is cold-drawn at a draw ratio of 3.83 between feed rollsand draw rolls, employing a snubbing pin of /2 inch diameter operatingat ambient temperature to localize the draw zone.

The resultant yarn has a denier of 210. The individual filaments have across section resembling that shown in FIG. 12 wherein the hollow coreoccupies about 12% of the over-all cross-sectional area of the filament.The filaments have a sparkling luster, particularly when observed afterfabriaction into a carpet sample, and have good resiliency. Microscopicexamination of the fibers reveals the presence of microfibers of thepolyester component, said microfibers having an average diameter of 0.18and average length of 165,u., there being about 9,500 of suchmicrofibers in a given cross section.

For purposes of comparison, the same spinnerette is employed for thespinning of a composition consisting entirely of the polycaproamideemployed above. The polymer temperature, extrusion rate, and all otherprocess conditions are maintained the same as above. Under suchconditions the die swell factoris only 1.5 and the extrudate streams donot coalesce to form hollow filaments.

By way of further comparison, the above experiment using the polymerblend composition is repeated using a spinnerette of essentially thesame configuration but wherein the minimum spacing between theindividual orifice is 0.002 inch. The filaments obtained have no hollowcore. Furthermore, after several hours of use, the polymer occludingareas within the orifice groups of the spinnerette are observed todistort or bulge outwardly. This causes spinning discontinuities andmakes it impossible to wipe the spinning face of the spinnerette, i.e.,to scrape it clean for restarting satisfactory extrusion.

Example 2 A sample of linear polyethylene having a density of 0.960 andmelt index of 2.0 (determined according to ASTM D1238-52T) is extrudedat 190 C. through a spinnerette having twenty seven groups of sixcircular orifices whose centers are equidistantly spaced on a circlehaving a diameter of 0.072 inch generally as shown in FIG. 2. Eachorifice has a diameter of 0.023 inch and a capillary length of 0.046inch. The diameter D of the occluding area is thus 0.049 inch. Theentrance angle to the capillary from the countcrbore is i.e., thecountcrbore has a fiat bottom. The polymer is extruded at a ratecalculated to give a capillary shear rate of 800 secsaid capillary shearrate being given by the formula where Q is the flow rate in m/sec., R isthe radius of the orifice in cm. and n is a constant expressing thedegree to which the flow deviates from the Newtonian condition. In thistest, n is taken as equal to 1.0. The die swell factor, measured on anorifice isolated from the regular groups is 3.0. The value of AD, is0.069 inch.

The extrudate is solidified by contact with a countercurrent flow ofheated nitrogen in a confining column. The rate of quenching is such asto maintain the polymer in molten or plastic form for at least 12 mm.downstream from the spinnerette. The solidified, hollow core filamentsare taken up on a winder positioned below the column.

The extrudates from the individual orifices of each group coalesced,forming an integral molten structure which upon cooling yields hollowfilaments. The yarn is drawn at a draw ratio of six at ambient roomtempera ture between standard feed rolls and draw rolls. The resultantdrawn filaments retain their hollow configuration, the area of thehollow interior being about 40% of the complete cross-sectional area ofthe filaments.

The die plate, after continued use, shows no evidence of bulging orrupture of the occluding area within the groups of orifices.

Example 3 A sample of Hoechst Hostalen GF-5200 polyethylene having amelt index of 0.4 (determined as in Example 2), and a density of 0.947is extruded at 220 C. through a spinnerette having 16 groups of threeorifices as shown in FIG. where each orifice has a width of 0.004 inch,and a cross-sectional area of 0.000128 square in. The distance ofseparation between the orifices is 0.004 inch. The diameter, D of thepolymer occluding area is 0.0304 inch, and the shape factor of theorifices is 5.6. The capillaries are perependicular to the extrusionface of the spinnerette and have a length of 0.030 inch. The polymer isextruded at a rate such that the capillary shear rate is 350 secr' theshear rate being determined from the equation of Example 2. Here Rdenotes the radius of a circular orifice of comparable area, and n isequal to 2.4. Under these conditions, a die swell factor of 3.3 isobserved. The value of AD is 0.013 inch. This swell factor issufl'icient to cause the polymer streams from the separate orifices tocontact each other and coalesce to form an integral structure whichyields a hollow fiber on cooling.

Example 4 A sample of polycaproamide having a melting point of 215 C.and relative formic acid viscosity of 50 is extruded at 250 C. through aspinnerette of the same general type employed in Example 3. Thespinnerette in this case, however, has 12 groups of three orificeswherein each orifice has a width of 0.005 inch, a cross-sectional areaof 0.000303 square inch and a capillary length of 0.018 inch. Thedistance of separation between the orifices is 0.006 inch. The diameterD of the polymer occluding area is 0.060 inch, and the shape factor ofthe orifices is 8.0. At an extrusion pressure of 5,000 p.s.i., a dieswell factor of 3.4 is obtained. The value of AD is 0.0175 inch. Underthese conditions coalescence of adjacent extrudate streams gives hollowfilaments. The filaments are cooled to solidification by countercurrentcontact with heated air, and are wound up at a rate of 1580 feet/min.The yarn is drawn at a draw ratio of 4 between feed and draw rolls usinga draw pin to localize the draw point. The drawn yarn retained acontinuous hollow cavity which occupied about 40% of the totalcross-sectional area. Fabrics woven from the drawn yarn in this exampleare light in weight, have high covering power, and additionally possessthe usual desirable characteristics of a nylon fabric.

Example 5 A sample of polyethylene terephthalate having a reducedorthochlorophenol viscosity of 0.83 is extruded at 12 270 C. through thespinnerette of Example 4. A die swell factor of 3.6 is obtained withproper adjustment of flow rate, and under these conditions satisfactorycoalescence is obtained to secure hollow filaments. The value of AD is0.018 inch.

Example 6 A sample of polycaproamide terminated with acetic acid, andhaving a relative formic acid viscosity of 52 with about 23milliequivalents of amine end groups per kilogram of polymer is extrudedat 260 C. at a throughput of 0.96 lbs/hr. through a spinnerette havingfour groups of trislots as shown in FIG. 8. Each orifice is 0.003 inchwide. The closest spacing between the orifices is 0.0035 inch. Theorifices are arranged about a circle having a diameter, D of 0.026 inch.Upstream from the spinnerette is a filter pack containing sand having anaverage particle size of 400 U.S. mesh.

The filaments are extruded into an atmosphere of air at a take-up speedof 1600-1625 feet per minute, followed by drawing at a draw ratio of3.4-to-1 at 1000 feet per minute.

The resultant drawn hollow filament has a denier of 41, with a tensilestrength of 4.8 am./denier. The hollow portion of the filament occupies911% of the cross-sectional area of the product filament.

The foregoing examples were presented for the purpose of illustratingthe melt-spinning process and hollow synthetic filaments of the presentinvention. It is, of course, understood that variations in theprocedures described in those examples as well as changes in thematerials used therein may be made without departing from the spirit ofthe invention and scope of the appended claims.

I claim:

1. A hollow filament consisting essentially of a sheath containing aninternal longitudinally extended cavity occupying at least 10 percent ofthe cross-section of said filament, said sheath comprising a continuousphase polyamide polymer containing dispersed therein a polyester in theform of microfibers lying predominantly in the dirrection of the axis ofsaid filament.

2. The filament of claim 1 wherein said microfibers have alength-to'diameter ratio greater than about 5.

3. The filament of claim 1 wherein the dispersed phase is a polyethyleneterephthalate and the continuous phase is a fiber-formingpolycaproamide.

References Cited UNITED STATES PATENTS 3,160,193 12/1964 Baggett et al152359 3,369,057 2/1968 Twilley 260857 3,382,305 5/1968 Breen16lMicrofiber Dig.

ROBERT F. BURNETT, Primary Examiner R. O. LINKER, JR., AssistantExaminer U.S. C1. X.R.

